1. Field of the Invention
The present invention relates generally to improvements in overhead projectors and in one aspect to an improved condensing lens construction to reduce glare from the stage of overhead projectors.
2. Description of the Prior Art
A particularly useful overhead projector, which is generally described in U.S. Pat. No. 3,126,786 comprises in optical alignment a transparent horizontal stage on which transparencies are laid, a source of intense illumination below the stage, an echelon lens structure comprising two annular echelon lenses disposed between the stage and the source of illumination, and a projection lens located above the stage. The echelon lens structure functions as a condensing lens, while the projection lens functions to form an image on a projection screen of the stage and of any transparency laid thereon. Usually a person who is using the projector stands at the side or rear of the projector so that he or she can conveniently change transparencies, point to significant portions of a transparency, or make marks on a transparency.
A problem with overhead projectors as described is that a region of concentrated glare is visible to the user when he or she looks at the transparency or uncovered stage. This glare, which causes eye strain and discomfort to the user, arises because a small portion of the light transmitted by the echelon lens structure is not controlled. FIG. 1 of the accompanying drawing (which is a fragmentary cross sectional schematic view of an annular echelon lens structure taken along a plane through the lens, which plane includes the optic axis of the lens), illustrates the path of controlled and uncontrolled light rays through two annular echelon lenses 11 and 12. Each annular increment or ridge 13 of the annular echelon lenses comprises a particonical working surface 14, a blind riser 15, and outer and inner edges, 16 and 17, at which the working surface and adjacent risers are joined. A ray of light 18 which is not reflected and does not strike the riser is said to be controlled, and the working surfaces are configured to refract all controlled light rays toward a substantially common focus and through the projection lens.
Examples of uncontrolled light rays in the plane illustrated in FIG. 1 are identified as light rays 20, 21, and 22. Light rays 20 and 21 are uncontrolled because they are reflected rather than refracted at interfaces between the air and the lens material. Light ray 22 is uncontrolled because it is diffracted at the edge 17. Due to the relatively small size of the source of illumination used in an overhead projector, all the uncontrolled (as well as controlled) light rays tend to be nearly coplanar with the optical axis of the echelon lenses 11 and 12 in the infinite number of planes extending radially from the optic axis. Thus, uncontrolled light rays lie in planes extending radially in all directions from the optic axis of the lenses 11 and 12. Within each plane uncontrolled light rays are moving in an infinite number of directions since the angle alpha (.alpha.) at which the uncontrolled rays emanate from the lens structure may have any value over the range 0.degree. to 180.degree..
When a user of the projector looks at the stage 24 of the projector or at a transparency on the stage, the user's eye "E" receives a light glare emanating from a propeller-shaped area illustrated by lines 28 the center of which is at the center of the lenses with the blades extending in directions in the same plane 25 as the eye of the user and the optic axis.
This glare is produced by the uncontrolled light emanating from the lenses in the plane of the observer and passing through the stage or transparency. As the user moves his position in a circle around the stage, as to a position E', he sees similar uncontrolled light in a different radial plane 27. In practice, no matter where the user stands, because of the annular symmetry of the lens the users eye will be in a plane with the optic axis and will see the glare resulting from the uncontrolled light.
In U.S. Pat. No. 3,511,563, a structural modification for reducing this glare in an overhead projector is described. This structural modification, which has been used with commercial satisfaction, includes an internally louvered film interposed between the stage and the condensing lens. The louvers are arranged in a spaced side-by-side relation and act as a set of "venetian blinds," which are substantially parallel to the path of controlled light rays and thus pass such light rays while blocking uncontrolled light rays that would otherwise travel to the user when in a normal user position.
There are at least three disadvantages in the described structural modification. First, it adds cost to the projector, especially when the louvered film is contoured as a shallow trough as is sometimes necessary for best results. Such a contouring slants the louvers at the outer edges of the film so the louvers are more closely parallel to the path of the controlled light rays; but the contouring of the film and the structure for supporting the contoured film add to the cost of the projector.
A second disadvantage is that the use of the film requires a greater distance between the condensing lens and the stage either to permit the contouring or to allow sufficient space between the condensing lens and the film in order to prevent Moire patterns on the screen.
The third disadvantage is that the louvers, the exterior faces of the louvered film, and the transparent material of the louvered film cause a reduction on the order of 15% to 40%, typically about 30%, in the overall transmission of controlled light through the projector. To compensate for this reduction in transmission of light, more powerful sources of illumination are used in the projector, leading to greater initial and replacement costs, greater cooling requirements, and greater electrical power consumption.
In summary, despite the great advantage of reduced glare provided by the louvered film, the named disadvantages lead to a desire for other ways to solve the problem of glare.